CWE-72: Improper Handling of Apple HFS+ Alternate Data Stream Path

What is Improper Handling of Apple HFS+ Alternate Data Stream Path?

• Overview: Improper Handling of Apple HFS+ Alternate Data Stream Path (CWE-72) occurs when a software product does not correctly manage or interpret special file paths related to the data or resource forks in the HFS+ file system, potentially leading to unintended behavior or security vulnerabilities.

• Exploitation Methods:

• Attackers can exploit this vulnerability by providing paths that specifically target the data or resource fork, leading the software to perform unintended actions.
• Common attack patterns include bypassing security checks or access controls by requesting only the data or resource fork of a file.

• Security Impact:

• Direct consequences of successful exploitation can include unauthorized access to file content or execution of unintended code.
• Potential cascading effects may involve escalation of privileges or spreading of malicious files within a system.
• Business impact includes possible data breaches, loss of sensitive information, and reputational damage.

• Prevention Guidelines:

• Specific code-level fixes involve ensuring that any file path handling logic explicitly accounts for and restricts access to data or resource forks unless explicitly required.
• Security best practices include thorough validation and sanitation of file paths and names, ensuring that access controls are applied consistently.
• Recommended tools and frameworks involve using updated libraries and APIs that are aware of HFS+ file system intricacies and automatically handle alternate data streams securely.

Corgea can automatically detect and fix Improper Handling of Apple HFS+ Alternate Data Stream Path in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

The Apple HFS+ file system permits files to have multiple data input streams, accessible through special paths. The Mac OS X operating system provides a way to access the different data input streams through special paths and as an extended attribute:

• Resource fork: file/..namedfork/rsrc, file/rsrc (deprecated), xattr:com.apple.ResourceFork
• Data fork: file/..namedfork/data (only versions prior to Mac OS X v10.5)

Additionally, on filesystems that lack native support for multiple streams, the resource fork and file metadata may be stored in a file with "._" prepended to the name. Forks can also be accessed through non-portable APIs. Forks inherit the file system access controls of the file they belong to. Programs need to control access to these paths, if the processing of a file system object is dependent on the structure of its path.

Vulnerable Code Example

Python Example

import os

def read_file_content(file_path):
    # Vulnerable to improper handling of HFS+ Alternate Data Streams
    # If file_path is something like "example.txt/..namedfork/rsrc", it can lead to unintended resource fork access
    with open(file_path, 'r') as file:
        content = file.read()
    return content

How to fix Improper Handling of Apple HFS+ Alternate Data Stream Path?

The vulnerability arises because the code does not validate or sanitize file paths that could exploit HFS+ alternate data streams by appending paths like "/..namedfork/rsrc". This can lead to unauthorized access to a file's resource fork, which might contain sensitive data or unexpected content.

To fix this issue:

1. Validate and Sanitize Input: Ensure that the file paths are sanitized to prevent alternate data stream paths. This can be achieved by normalizing paths and checking for unexpected segments.
2. Use Whitelisting: Implement path whitelisting where only known safe paths are allowed.
3. Restrict File Access: Limit the directories and files that can be accessed by the application to a specific set of known safe files.

Fixed Code Example

import os

def read_file_content(file_path):
    # Normalize the file path to prevent exploitation of HFS+ alternate data streams
    normalized_path = os.path.normpath(file_path)

    # Ensure that the path does not contain unexpected segments like "..namedfork/rsrc"
    if "..namedfork" in normalized_path:
        raise ValueError("Invalid file path. Access to alternate data streams is prohibited.")

    # Restrict access to a specific directory (e.g., "/safe_directory")
    allowed_directory = "/safe_directory"
    absolute_path = os.path.abspath(normalized_path)
    
    # Ensure the path is within the allowed directory
    if not absolute_path.startswith(os.path.abspath(allowed_directory)):
        raise ValueError("Access to files outside the allowed directory is prohibited.")

    with open(absolute_path, 'r') as file:
        content = file.read()
    return content

In the fixed code:

• We normalize the file path using os.path.normpath() to ensure that the path is in a standard format.
• We check for the presence of suspicious segments such as "..namedfork" in the path and raise an error if found.
• We convert the normalized path to an absolute path and ensure it starts with the absolute path of the allowed directory. This prevents directory traversal and ensures files are accessed only within a specified directory ("/safe_directory").